The present invention generally relates to noise suppression apparatuses, and more particularly to a noise suppression apparatus for suppressing a noise in a voice recognition apparatus which is used in measurements, robots and the like.
When picking up a voice (speech) under a noisy condition, it is necessary to extract a voice component from an input signal which includes both an audio signal and a noise component. However, there still does not exist a system which can easily and completely separate the audio signal and the noise component.
As methods of picking up the voice, there is a single input system and a plural input system which includes a double input system and the like. According to the single input system, no voice is picked up and only the noise component is initially picked up so as to analyze the noise component by a learning function. An inverse filter is designed based on the analyzed noise component, and the input which includes the audio signal and the noise component is passed through this inverse filter so as to improve a signal-to-noise (S/N) ratio of the input signal. Such a system is disclosed in a Japanese Laid-Open Patent Application No. 54-147708, for example.
However, the system according to the Japanese Laid-Open Patent Application No. 54-147708 requires both fast-Fourier-transform (FFT) and inverse FFT to constitute the inverse filter, and as a result, the operation is complex and the scale of the system as a whole becomes large.
On the other hand, according to the plural input system, a main microphone is used for picking up the voice and one or more reference microphones are used for picking up the noise component. When the noise component is simply subtracted from the input signal outputted from the main microphone, the operation is extremely simple but the noise eliminating effect cannot be obtained for a large frequency band because of the different phase characteristics of the microphones.
Hence, a Japanese Laid-Open Patent Application No. 56-115000 discloses a method of obtaining a correlation coefficient between the input signal from the main microphone and the signals from the reference microphone and varying a subtraction constant. But even according to this method, the noise eliminating effect is small despite the extremely complex operation, and this method is unsuited for practical use.
When the noise cannot be suppressed satisfactorily in the speech recognition apparatus, there is a problem in that the accuracy with which the voice recognition is made becomes poor.